Recently, wireless relay networks have become an area of popular research. This is due to their ability to extend communication coverage, increase channel capacity, and improve link reliability. There are two types of relaying strategies: (1) decode-and-forward (DF) relay, whereby a received signal is decoded, re-encoded, and forwarded by a relay station, and (2) amplify-and-forward (AF) relay, whereby a received signal is simply amplified and forwarded to another wireless device or another relay station in the network. The DF scheme is generally used in dedicated relay stations (e.g. relay stations operating in 3GPP LTE-Advanced and IEEE 802.16j systems) which can afford the complexity involved under the DF scheme. However, the AF scheme is generally preferred for battery operated devices as such devices usually have their own data to transmit and the complexity of relaying operation needs to be kept to a minimum (e.g. nodes in smart grid and sensor networks).
The AF relay scheme can be further categorised into fixed gain amplification and variable gain amplification. The difference between these two categories is that fixed gain amplification is based on the average received channel power while variable gain amplification is based on the instantaneous received channel power. In a multi-hop AF network with a single relay path, the variable gain AF relaying scheme significantly outperforms the fixed gain AF relaying scheme. However, in the variable gain AF relaying scheme, channel estimation needs to be performed by each of the relay node in order to determine the instantaneous channel state information (CSI). Such a relay node is also commonly referred to as a CSI-assisted AF relay. Therefore, compared to a fixed gain AF relay node, the computation complexity at a variable gain AF relay node is significantly higher due to the need for channel estimation. Fixed gain AF relaying scheme (also generally known as non-regenerative relay scheme) has been widely applied in low complexity systems, such as emerging energy and utility management applications as well as industrial wireless sensor networks.